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Hydraulic procedures are required to compute flow characteristics in closed conduits and open channels during the passage of a runoff hydrograph. In all cases, hydraulic calculations are aimed at computing energy losses in the flow conveyance system such that the hydraulic grade line (HGL) of the system can be established. The HGL is often used to determine the location of the hydrostatic head along the water conveyance system. In the case of open channel flow, the HGL generally coincides with the water surface. In closed conduit flow conveyance systems, flow conditions can be pressurised and water rises to the HGL level at junction pits. The design process is commonly a series of iterations involving modifying the various hydraulic structures such that the resulting HGL complies with the design standards specified. These design standards can include:
In closed conduit flow conditions, calculation of energy losses within the flow conveyance system is most commonly based on applying energy loss factors to the velocity head (v2/2g). The energy loss attributed to pipe friction is best calculated by the Darcy-Weisbach Equation. Localised energy losses due to junction and inlet pits are computed in a similar manner using standard coefficients derived from extensive laboratory tests of these structures.
The simplest model available for computing flood levels and flow velocities is the use of empirical formulae such as the Chezy Equation and the Manning's Equation. These equations are referred to as slope-area methods on the basis that they utilises simple relationships between the discharge in a river to the energy gradient, flow cross-sectional area and hydraulic roughness. These formulae are suited for computing water levels only at a single location and for a single discharge value (i.e. steady flow assumptions apply) under uniform flow conditions.
These methods at best would address the second of the above five design considerations related to the hydraulics behaviour of the flow conveyance system. Often the use of slope-area methods are inappropriate due to the fact that natural river flows are not uniform.
Open channel flow conditions can often be categorised as either rapidly varying or gradually varying. Rapidly varied flow occurs whenever there is a abrupt change in the geometry of the channel or in the flow regime of the flow. In regions of rapidly varied flow, the water surface profile changes rapidly. Examples of rapidly varying flow include flow over weirs and through regions of rapid changes in bed elevation or channel width (i.e. abrupt change in geometry) and hydraulic jumps (i.e. change in flow regime). Simulations of rapidly varied flow conditions require the solution to the equations of conservation of mass and conservation of momentum in fluid flow (i.e. the Saint Venant Equations).
The flow condition generally occurring in natural river and floodplain systems can be categorised as that of gradually varied flow; that is, conditions in which the flow characteristics are non-uniform but vary gradually with distance along the channel due to gradual variation of bed slope, channel geometry and hydraulic roughness.
A number of river and floodplain computer models are available which solve either the energy equation (for steady gradually varied flow only) or the Saint Venant Equations either in its complete form or its simplified form. River and floodplain models can be categorised into the following groups:
The applicability of each of the models listed is very much dependent on such factors as:
The sources of some of the above models are as follows:
For localised energy losses associated with junction and inlet pits, the term fL/D in equation (3.1) is replaced by a head loss coefficients derived from standard charts resulting from various laboratory studies, particular the "Missouri Charts" which were based on extensive laboratory studies by Sangster et al. (1958) from the University of Missouri.
Steady flow models are normally one-dimensional, although models such as HECRAS have some quasi-two-dimensional capabilities with 'split-flow' option.
Steady flow one-dimensional models can also be used to model flow characteristics in a network system or a braided channel system. Models such as HECRAS can be used in a fairly creative manner to compute flood levels in reasonably complex river networks.
Field data requirements for this type of model are river cross-sections and dimensions of special structures (bridges, culverts, weirs, etc.) likely to cause backwater effects. The main model parameters are the roughness coefficients, and flow contraction and expansion coefficients. In HECRAS, the use of the split flow option would also require weir flow coefficients for the overtopping sections.
Models such as MIKE-11, DYNHYD and CELLS fall into this group of unsteady flow models. MIKE-11 has the option of removing or reinstating the local acceleration term.
In a situation where the shape of the flood hydrograph is 'peaky', that is when flow conditions are rapidly varied (such as in an road embankment failure event), the additional local acceleration term becomes significant. Models such as DAMBRK and MIKE-11 are equipped to model such situations.
There is a further type of quasi two-dimensional model for water quality modelling in rivers. These models are 'two-dimensional' in the vertical axis to model such stratified flow phenomena as salt intrusion. Such models include MIKE-12 and TIDEWAY-2DV.
Data requirements for this type of model are similar to those for the steady flow models. If the quasi two-dimensional approach is adopted, additional survey data on the ground profile at the lateral flow connections and dimensions of the side channel will be needed.
Figure 3.1 Typical Output from MIKE-21
Three-dimensional models are not often applied to river and floodplain studies and are mainly used in ocean and estuary studies. They feature in water quality modelling investigation where variation of water pollutants is dependent on flow distribution (x and y directions) and density (z direction).
Australian Rainfall And Runoff;(1987), A Guide to Flood Estimation, Volume I & II.
Sangster, W.M., Wood, H.M., Smerdon, E.T. and Bossy, H.G., (1958), Pressure Changes at Storm Drain Junctions, Engineering Series Bulletin No. 41, Engineering Experiment Station, University of Missouri.